Servo motor driven r.f. attenuator

ABSTRACT

A system for cancelling or suppressing the amplitude of the carrier and sideband signals of a radio transmitter to facilitate analysis of spurious and noise outputs from the transmitter is disclosed. The system includes means for deriving a carrier cancelling signal from an actual sample of the transmitter output, signal processing circuitry and summing means. Also, an electromechanical signal controller is disclosed capable of continuously correcting the amplitude of the cancellation signal by a selected ratio with respect to the transmitter test signal.

United States Patent 191 Martin et al.

[ June 19, 1973 1 1 SERVO MOTOR DRIVEN R.F.

ATTENUATOR v [76] Inventors: David R. Martin; Walter A. Sauter,

both of 0/0 John E. Wagner, 1041 East Green Street, Suite 202, Pasadena,Calif. 91106 [22] Filed: Sept. 22, 1971 [21] Appl. No.: 182,810

Related U.S. Application Data [63] Continuation-in-part of Ser. No.65,561, Aug. 20,

1970, Pat. No. 3,648,176.

[52] U.S. Cl. 318/16, 318/618, 338/174,

[51] Int. Cl. H04q 9/00 [58] Field of Search 318/618, 629, 16,

[56] References Cited UNITED STATES PATENTS 3,114,869 12/1963 Goodwin..318/6l8X 3,343,115 9/1967 Greenwood 338/174 1,672,414 6/1928 Horton i338/174 X 2,514,682 7/1950 Tellkamp.... 338/174 X 2,606,984 8/1952Burgess 338/174 X Primary ExaminerT. E. Lynch Attorney-John E. Wagner 57ABSTRACT A system for cancelling or suppressing the amplitude of thecarrier and sideband signals of a radio transmitter to facilitateanalysis of spurious and noise outputs from the transmitter isdisclosed. The system includes means for deriving a carrier cancellingsignal from an actual sample of the transmitter output, signalprocessing circuitry and summing means. Also, an electromechanicalsignal controller is disclosed capable of continuously correcting theamplitude of the cancellation signal by a selected ratio with respect tothe transmitter test signal.

4 Claims, 10 Drawing Figures Patented June 1 9, 1973 5 Sheets-Sheet}FIG.2

46C? 6 KHz 53 lO'KHz 5 46E 200 KHt Patented June 19, 1973 5 Sheets-Sheet5 Ptented June19, 1973 3,740,627

5 Sheets-Sheet 4 Patented JuIIIe 19, 1973 5 Sheets-Sheetfi RF AWL RFANPL Gigi:

FIG. 7

RF "7 SWITCH .|RF AMPL h m AMPL l KHZ DENOD I KHZ MVB FIG, IO

SERVO MOTOR DRIVEN R.F. ATTENUATOR CROSS REFERENCE TO RELATEDAPPLICATION BACKGROUND OF THE INVENTION The need to measure the spuriousand noise outputs of high frequency/very high frequency/ultra-highfrequency (I-IF/VI-IF/UHF) transmitters from :1 channel width to MHzfrom the tuned carrier frequency heretofore has posed a difficultmeasurement problem because of the large dynamic range betweenthecarrier, sideband, and spurious output levels. Usually, the

. sideband power is more than 100 dB below the carrier level at :5channel widths. The dynamic range of pres- 'ent spectrum analyzers usedfor sideband splatter mea- ,measurements of spurious and noise outputsis difficult when the sideband power is more than 70dB below the carrierlevel.

Heretofore, adjacent channel spurious signal measurements have been madeonly to the extent that passive filtering techniques will allow. Thebasic concept of cancellation of a selected carrier signal by adjustingthe amplitude and phase of a correction signal and combining it with thecarrier is known in the art; Our

copending patent application, Ser. No. 799,781, filed Feb. 17, 1969,also discloses a concept for cancelling an unwanted signal'by thederivation of two compo- ,nents, of the undesired signal, adjustingtheamplitude of the components by a ratio, and adding the adjustedcomponent signals to the unwanted signal, similar to this invention. v

BRIEF STATEMENT OF THIS INVENTION We have employed the basic twocomponent control concept of our above-identified invention and produceda signal cancellation system capable of not only eliminatinginterference from adjacent transmitters but which is able to cancelthecarrier and modulation of an'operatin g transmitter in order to measureits spurious outputs.

The system in accordance with this invention employs not only theconcept of our above-identified application, but also includes aself-referenced tracking loop-with 'both velocity and accelerationfeedback in the electromechanical signal controller. 7

The system includes means for sampling a transmitter sigiial developingtheir sine, and cosine components, an electromechanical signalcontroller for introducing an amplitude adjustment to each of thecomponents, and means for combining the modified amplitude componentswith the sampled signal to substract the carrier signal and itsmodulation. Additionally, this system employs selective delay meanswhich introduce a controlled amount of relative delay into the testsignal whereby the carrier and to a'variable extent the modulation iscancelled. I

A novel signal controller employs a rf potentiometer directly coupled toboth a drive motor and a velocity generator whereby corrections in thepotentiometer position may be made automatically by driving the motorand the speed or movement of the controller can be monitored and used asa velocity and acceleration feedback control.

BRIEF DESCRIPTION OF THE DRAWINGS Our invention may be more clearlyunderstood from the following detailed description and by reference tothe drawings in which:

FIG. 1 is a simplified block diagram of the signal cancellation systemof this invention;

FIG. 2 is a simplified block diagram of the circuitry for synthesizing acarrier cancelling system;

FIG. 3 is a simplified interconnection diagram of this system in use;

' FIG. 4 is an electrical schematic diagram of the embodiment of FIG. 3;

FIG. 5 is an electrical schematic diagram of the servo amplifiers ofFIG. 4;

. FIG. 6 isaperspective view'of the motor, potentiometer, tachometerassembly of FIG. 4 with portions-broken away; I v I FIG. 7 is a blockdiagram of the summer/amplifier/detector assembly of FIG. 4;

. FIG. 8 is an electrical schematic diagram of the demodulators of FIG.7;

FIG. 9 is an electrical schematic diagram of the synchronousdetector ofFIG. 7; and,

FIG. 10 is an electrical schematic of the RF switch of FIG. 7.

DETAILED DESCRIPTION OF THE'INVENTION The essential principle ofoperation of this invention A is based on the fact that the output of aradio transmitcarrier of the signal being received at the measurementpoint. The subtraction of this appropriately controlled reference signalsample is accomplished in real time.

It is evident from the theory of such a signal cancellation system thatthe amplitudes and phases of the .cancelling signal and test signal mustbe very nearly the same. Thus, for example, their phases must be within10" radian to reduce the interference by 60 dB, even when theiramplitudes are identically the same. The control of the phase of asignal with this degree of precision often poses insurmountable physicalproblems. However, a signal whose amplitude and phase vary arbitrarilywith time can be uniquely transformed into two signal components whoseamplitude ratios only change with time. By maintaining precise controlof these amplitude ratios, thisinvention synthesizes a cancellationsignal with a degree of precision not attainable through independentamplitude and phase controls. The amount of relative cancellation of thesideband and spurious frequencies is adjustable via the channel widthswitches. Each switch adds enough coaxial cable to the test signal pathlength to cause the amount of cancellation at one channel width awayfrom the carrier to be down by at least 3 dB due to the associatedelectrical phase error.

Now referring to FIG. I, the basic principle of the invention may beseen. It includes as the basic element a pair of signal controllers and11 into which a reference signal, in this case a sample of the output ofthe radio transmitter to be analyzed, is applied over lead 12 while thesame signal from the transmitter under test and unshown in the drawingis applied to a coupler 13 via lead 14. Carrier cancelling signalsproduced by the signal controllers 11 and 10 which are derived in themanner described below are applied to the coupler 13 over lead 15. Thesummation of the transmitter signal on lead 14 and the correction orcancellation signal of lead 15 appears on lead and thence passingthrough a second coupler 21 to an output lead 22. In normalcircumstances the lead 22 is connected to the test signal input to aspectrum analyzer, not shown in the drawing. The correction systemconstitutes a tracking loop by reason of the application of thecorrected signal via lead 23 to the control circuitry for the signalcontrollers l0 and 11. This control circuitry includes a pair ofsynchronous detectors 24 and 25, DC amplifiers 26 and 27 and integratoramplifiers 28 and 29. Synchronism for the entire correction system isobtained from the transmitter over leads 90 and 91.

Operation of the system as disclosed in FIG. 1 is as follows:

The reference signal on lead 12 is shifted in phase and changed inamplitude by the pair of signal controllers l0 and 11. The change ofamplitude, k, and the phase shift, I are so adjusted that the output ofthe signal' controllers yields a spectrum identical to that needed forthe cancelling signal. The amplitude factors k of the respective signalcontrollers 10 and 11 is developed from the dc content of demodulatedcomponents Sa and Set 90 of the residual signal after summing on lead23. The phase change, 1 corresponds to an equilibrium time delay, 1',such that the cancellation is not restricted to one carrier frequency.

Before an equilibrium condition is reached or during the period wherethe carrier frequency within the channel changes, the values of k and Iwill not be exactly the same as required. To obtain a high degree ofcancellation capability, therefore, one must continuously adjust thesevalues such that correct values of k and I are obtained at all times.This correction is achieved by high gain, automatic tracking loopsincluding couplers 13, 21, synchronous detectors 24 and amplifiers26-29. Employing this invention, the functions k and I are notordinarily time varying functions. Therefore, the signal synthesisprocess necessary for the operation of the system does not require thesetwo parameters to change at the rf frequency. Furthermore, since k and Dchange very slowly, the maintenance of the uniform cancelling signal ona continuous basis, becomes considerably simplified. Also, the problemof the usual time lag between the synthesization of the cancellationsignal and its subtraction from the test signal is completely avoided inthis approach.

Next referring to FIG. 2, a simplified block diagram of an actualembodiment of this invention is shown therein. The same functionalelements appearing in FIG. 1 are given identical numbers in this andsucceeding figures of the drawing for clarity. In this embodiment, thesystem is designed to cancel thecarrier and modulation for radiotransmitters operating in the amplitude, frequency, pulse code, singlesideband or continuous wave modes of operation. This is possible bymerely the addition of selected delay in the branch 14 by means ofselector switches 41 through each providing either a through conductivepath therethrough or a delay line of different length for'each switchoperated. Typical delays of 2.4 to 430 nanoseconds are achieved byrespective lengths of coaxial cable for each of the switches astabulated below:

switch 4| 300 RG 558/ U coaxial cable In the embodiment of FIG. 2,provision is additionally made for determining the absolute level ofsignal measurements by the (selective) introduction of a calibrationsignal of known level into the system in parallel with the test signal.This is accomplished through the addition of a summer 50 and a dividercircuit 51 at the input end of the system with the normal test signalintroduced over lead 12 as shown in FIG. 1 and the calibration inputapplied to the summer 50 over lead 40.

In the embodiment of FIG. 2, time quadratured samples of the test signale, are applied to the signal controllers 10 and 11 from a pair ofquadrature couplers 52 and to synchronous detectors 24 and 25 via leads53 and 54. The function of the signal controllers 10 and 11 is to setthe amplitude ratios of the in-phase (sine) and quadrature (cosine)components of the sample signals such that suppression of the carrierfrequency can be secured.

The signal controller ratios are set by the two synchronous detectors 24and 25 circuits, one each for the sine and cosine components. The inputsto each of the synchronous detectors 24 and 25 are:

l. A reference signal over respective lead 53 or 54 in-phase withitssignal controller output signal.

2. An amplified sample of the residual signal e e, after summing.

When the amplified residual signal sample e e, lead 60 from divider 21is applied to the sine and cosine syn chronour detectors, the sine andcosine components of the residual carrier signal are indicated at theoutput of their respective synchronous detectors 24 or 25 byproportional DC voltages. These DC error voltages drive their respectiveamplifiers and signal controllers 10 and 1]. The ratio setting of thesignal controllers 10 and 11 will increase or decrease until themagnitude. of each carrier signal component ratio setting.

A multivibrator and RF switch unshown in block diagram FIG. 2 but shownin FIG. 7 provide effective chopper stabilization of the amplifiers 55and 56 and eliminate DC offset effects of the synchronous detectors.

The two complementary quadrature couplers 52 are used to shift the phaseangle of the reference signal applied to signal controllers 10 and 11 byrelative to the other. One unit covers one frequency range such as 3-35MHz. These quadrature couplers 52 consist of two hybrid junctions. Theircollateral ports are coupled together with two compensated phase shiftnetworks. Although the absolute phase shifts through the two networksshift with frequency, their output phase difference is always 90.

The function of the system of this invention is to subtract the carrierand major sidebands of one sample of a transmitter output signal fromanother so that the weaker residues of spurious and noise can bemeasured with a spectrum analyzer such as the Model 85lB/8551B producedby Hewlett-Packard Company of Palo Alto, California. This isaccomplished by controlling the phase and amplitude of the carrier ofone sample of the transmitter output signal so that it is identical inamplitude to, but phase shifted by 180 degrees with respect to thecarrier of another sample of the transmitter output signal. Summing ofthese signals will result in cancellation of the carrier signal and itssidebands. With matched phase lengths the weaker residues of spuriousand noise in adjacent channels would normally also be suppressed. Theamount of suppression at one or more channel widths from the carrier iscontrolled by adding or removing lengths of coaxial cable 46 A-E withthe channel width switches 41-45.

The use of differential lengths of coaxial cable to generate specificphase angle offset errors for specific differential offset frequencies(channel widths) has the advantage of being independent of the carrierfrequency. The system will adjust the reference signal to any arbitraryamplitude and phase angle to effect the required cancellation of thecarrier. This capability is independent of the number of carrierwavelengths in the differential path length. An offset or modulationfrequency will have a phase angle offset because it will arrive at thesumming point late. The resultant electrical phase angle error isdirectly proportional to the differential path length.

The length of additional coaxial cable required to provide a given phaseangle offset can be calculated v of the system. A calibration signalgenerator 103 furnishing a standard amplitude signal is connected to thesystem 100 via lead 40. The selectable delay cables 46A-E of FIG. 2appears as delay assembly 146 connected to the system via cable 14. Theoutput of the system 100 constituting the transmitter output signal lesscarrier and modulation appears on lead 22 for introduction into aspectrum analyzer 104 which is then capable of analyzing any noise orspurious output of the transmitter 101 without overloading andsubmerging of those signals by the carrier.

Now refer to FIG. 4 which constitutes a schematic of the embodiment ofthe system 100 of FIG. 3. In this FIG. 4, certain of the inventivefeatures of the system are shown and are described below. FIG. 4 showsthe front panel terminals and controls. They include the input terminal12 for test signal and terminal 40 for a calibration signal if desiredas well as an output terminal on lead 22 designed to be connected to aspectrum analyzer. A jumper 22a and lead 22 for allowing the selectionin low level or high level 20 db) output to the spectrum analyzerappears in the front panel.

Selector switches 41 through 45 for setting delay time appear in thefront panel with switch 42 in its actuated position whereby delay line46b is inserted in the circuit. One additional selector switch forfrequency range selection is shown coupled to a switch 71 in theoperating circuit of three relays 72, 73 and 74. In the position shown,the switch 71 is in the low frequency position, for example, 3-35 MI-Iz.In the opposite position with relays 72, 73 and 74 actuated, the systemis in its high frequency range, for example, 35-425 MHZ.

One additional front control switch 75 serves to disable the servo loop.As shown, switch 75 is in the normal operating condition. With theswitch disabled, the servo loop is disabled. The servo loop includes aservo amplifier 30 connected via lead A to the switch 75 and by lead 76to a motor 80 of the sine signal control 10. Servo amplifier 31 isconnected via B to the switch 75 and to a motor 81 of the cosine signalcontroller 11.

The signal processing within the system of FIG. 4 is as described inconnection with FIG. 3 and the same designations are applied in FIG. 3to the same functional elements. Incoming signal from the testtransmitter at terminal 12 is summed in the summation circuit 50 withthe calibration input over lead 40 as described above and divided intotwo signal, the first of which is a reference signal passed throughrelay 72 and one or the other of quad couplers 32 and 33 which developtwo quadrature components of the incoming signal. These quadraturecomponents are applied through relays 73 and 74 to divider circuits 21aassociated with the sine and cosine components respectively. Thesedividers 21 apply sine and cosine inputs to hybrid couplers 85 and 86associated with respective signal controllers 10 and 11. The dividersalso apply sine and cosine inputs over leads 90 and 91, respectively, tothe summer amplifier and detector circuit 92, the latter of which isshown in more detail in FIG. 7. Signals reaching the hybrid 85 areapplied via leads and 111 to opposite ends of an rf potentiometer l 12forming a part of the signal controller 10. Signals reaching hybrid 86are applied via leads 113 and 114 to opposite ends of an rfpotentiometer 115 of signal controller 11. Both rf potentiometers 112and 115 have center taps grounded whereby signals may be attenuated andreversed in polarity while passing through them. The potentiometer 112is mechanically coupled directly to the shaft of the motor 80. Also,directly coupled to the shaft of motor 80 is a tachometer 116' orsimilar motor operatedas a generator to provide a velocity relatedsignal on either lead 120 or 121 depending upon the direction of therotation of the motor 80. The velocity signal on lead 120 or 121 isapplied to the input of the servo amplifier 30 constituting a velocityfeedback loop in the signal controller circuit.

The rf potentiometer 115 is similarly mechanically coupled directly tothe motor 81 and to tachometer 122 which provides velocity feedbacksignals over leads 123 or 124 to the servo amplifier 31 thus providingcosine signal controller velocity feedback as well. The mechanicaldetails of the signal controllers l0 and 11 appear in FIG. 6 to whichreference is now made ously variable reversible polarity potentiometer,the

electrical phase of which is constant independent of pick-off position.Employing this invention, this electromechanical device as shown in FIG.6 is capable of responding to required changes in correction signal tocontinuously maintain a correction signal in proper magnitude and phaseto provide effective cancellation of the carrier. This is accomplishedthrough both the servo loop tracking of this system via the servoamplifier and velocity feedback from the tachometer 116.

Now refer to FIG. which discloses a circuit which is particularlysuitable for the servo amplifier of FIG. 4. It employs two pairs ofdirectly coupled transistors in a complementary symmetry configurationwith input taneously. A positive drive voltage turns the transistor 161off and when the drive voltage reaches zero, the transistor 161 isturned on and signals are conducted between terminals 162 and 163 withinsignificant losses. The foregoing specific circuits described areillustrative only of the particular circuits found to operatesuccessfully as a part of the system of this invention. It is recognizedthat other specific circuits may be substituted and interconnected inthe manner taught herein to obtain the advantages of this invention.

When carrying out the teaching of this invention, the system providesmeans for accurately measuring electromagnetic spectral characteristicsat HF, VHF, and UHF with an amplitude dynamic range as great as 120 dB.In practice, the system can be used with spectrum analyzers to expandeffectively their dynamic range from a nominal 70 dB to 120 dB. Some ofthe special features associated with the measurement system, asdeveloped, are as follows:

a. The system employs no band filtering for operation and so has a broadband capability, where a frequency band ranging from 3 to 425 MHz (aratio of 108:1) can signals from the demodulators applied to the base ofv the transistors of the input stages through an input amplifier andwith feedback from the tachometer output of the motor-tachometerassemblies also applied to the input amplifier. The output of the commonemitters constitutes the control input to the motor of FIGS. 4 and 6.

FIG. 7 shows in block diagram form the circuit arrangement of the summeramplifier detector assembly of FIG. 4.

The circuit 92 constituting FIG. 7 comprises basically the summer 13,divider 21, and demodulators 24 and 25 with their associated amplifiersas shown in FIG. 2. The details of the demodulators 24 and 25 may beseen as including a free running multivibrator 140 operating forexample, at lKI-lz and driving the RF switch 141. This RF switch 141modulates the amplified combined signal from divider 21 and passes itthrough another RF amplifier which drives synchronous detectors 142. Thesynchronous detectors 142 detect the residual signal and provide thein-phase and quadrature error signals.

' These signals are then amplified by and then demodutor of FIG. 7applied to the gate electrode and the de- 7 modulated signal extractedfrom the parallel connected drain electrodes.

The synchronous detectors 142 of FIG. 7 may be seen in FIG. 9 as simpledouble balanced mixers such as the type FC 200R produced by the LorchCo. of Englewood, New Jersey.

The RF switch 141 of FIG. 7 is illustrated in FIG. 10

be accommodated in only two switching steps,

b. Through the useof selective delay switching the system canaccommodate a wide range of channel widths,

e. The system is suited for a wide range of modulations within thechannel during adjacent channel measurements,

d. It provides external calibration capability for quan titativemeasurements of the amplitudes in the spectrum, v

e. The system inherently includes an automatic tracking capability toaccommodate any change of the carrier. frequency in the channel,

f. The system employs linear signal processing so it does not distortthe spectral characteristics of the signal even for small amplitudes.

In the foregoing description of this invention, the primary andpreferred application has been described, that is, in conjunction withthe analysis of the output of the radio transmitter using a spectrumanalyzer. It must be recognized, however, that the use of this inventionis in no way limited to spectrum analyzer applications nor for thatmatter to radio transmitters. In particular, any communication systemhaving a predominant carrier or signal and subject to spurious signalsbelow the level of the carrier can benefit from this invention.Therefore, in the reading of the foregoing specification, keep in mindthat the source of signal which is processed with this invention and itsultimate utilization device may be significantly different from theradio transmitter and spectrum analyzer disclosed without departing fromthis invention. It is further recognized that one skilled in the artmight by simple modifications vary the particular arrangement ofelements making up this invention as described above. Therefore, thepatent granted hereunder is not limited to the specific disclosure butto the invention as described in the following claims and theirequivalents.

as comprising a grounded gate N channel field effect We claim:

1. A variable attenuator comprising,

a resistive element comprising a planar interrupted circular ring track,

a conductive element comprising a 'planar interrupted circular ringtrack,

said resistive and conductive elements being of equal diameter and inspaced coaxial relationship,

common wiper means electrically connecting said resistive and conductivetracks,

a shaft for mounting said common wiper means for rotatable movementaround the common axis of and along said resistive and-conductivetracks,

motor means coupled directly to said shaft for driving said common wipermeans, and

generator means coupled directly to said shaft for producing anelectrical signal responsive to movement of said shaft.

2. The combination in accordance with claim' 1 ineluding a first pair ofterminals connected to said generator; a second pair of terminalsconnected to opposite ends of said resistive track and an additionalterminal connected to said conductive track whereby said first pair ofterminals provides a voltage rate related to wherein said resistive andconductive tracks are each planar interrupted rings and said commonwiper means is a radially extended bifurcated arm with substantiallyequal signal distance therethrough.

4. The combination in accordance with claim 1 wherein said conductiveand resistive tracks are in par- I allel side by side relation and ofsubstantially equal length whereby the signal path length remainssubstantially constant through the attenuator regardless of the positionof said common wiper means.

1. A variable attenuator comprising, a resistive element comprising aplanar interrupted circular ring track, a conductive element comprisinga planar interrupted circular ring track, said resistive and conductiveelements being of equal diameter and in spaced coaxial relationship,common wiper means electrically connecting said resistive and conductivetracks, a shaft for mounting said common wiper means for rotatablemovement around the common axis of and along said resistive andconductive tracks, motor means coupled directly to said shaft fordriving said common wiper means, and generator means coupled directly tosaid shaft for producing an electrical signal responsive to movement ofsaid shaft.
 2. The combination in accordance with claim 1 including afirst pair of terminals connected to said generator; a second pair ofterminals connected to opposite ends of said resistive track and anadditional terminal connected to said conductive track whereby saidfirst pair of terminals provides a voltage rate related to velocity ofsaid shaft output and said second pair of terminals and said additionalterminal provide resistance values representative of the position ofsaid shaft and substantially uniform phase shift of signals between saidsecond pair of terminals and said additional terminal.
 3. Thecombination in accordance with claim 1 wherein said resistive andconductive tracks are each planar interrupted rings and said commonwiper means is a radially extended bifurcated arm with substantiallyequal signal distance therethrough.
 4. The combination in accordancewith claim 1 wherein said conductive and resistive tracks are inparallel side by side relation and of substantially equal length wherebythe signal path length remains substantially constant through theattenuator regardless of the position of said common wiper means.